Over the years, Keith McCarthy has become used to a certain way of doing things on his computers, which, like most others on the planet, have long run on Microsoft’s Windows software. But last week, when he got his hands on a laptop running the newest version of Windows for the first time, McCarthy was flummoxed.

Many of the familiar signposts from PCs of yore are gone in Windows 8, like the Start button for getting to programs and the drop-down menus that list their functions.

It took McCarthy several minutes just to figure out how to compose an e-mail message in Windows 8, which has a stripped-down look and on-screen buttons that at times resemble the runic assembly instructions for Ikea furniture.

“It made me feel like the biggest amateur computer user ever,” said McCarthy, 59, a copywriter in New York. Windows, which has more than a billion users around the world, is getting a radical makeover, a rare move for a product with such vast reach. The new design is likely to cause some head-scratching for those who buy the latest machines when Win 8 goes on sale this Friday.

To Microsoft and early fans of Windows 8, the software is a fresh, bold reinvention of the operating system for an era of touch-screen devices like the iPad, which are reshaping computing. Microsoft needs the software to succeed so it can restore some of its fading relevance after years of watching the likes of Apple and Google outflank it in the mobile market.

To its detractors, though, Windows 8 is a renovation gone wrong, one that will needlessly force people to relearn how they use a device every bit as common as a microwave oven.

“I don’t think any user was asking for that,” said John Ludwig, a former Microsoft executive who worked on Windows and is now a venture capitalist. “They just want the current user interface, but better.”

Little about the new Windows will look familiar to those who have used older versions. The Start screen, a kind of main menu, is dominated by a colorful grid of rectangles and squares that users can tap with a finger or click with a mouse to start applications. Many of these so-called live tiles constantly flicker with new information piped in from the Internet, like news headlines and Facebook photos.

What is harder to find are many of the conventions that have been a part of PCs since most people began using them, like the strip of icons at the bottom of the screen for jumping between applications. The mail and calendar programs are starkly minimalist.

It is as if an automaker hid the speedometer, turn signals and gear shift in its cars, and told drivers to tap their dashboards to reveal those functions. There is a more conventional “desktop” mode for running Microsoft Office and older programs, though there is no way to permanently switch to it.

Microsoft knew in the summer of 2009 that it wanted to shake up Windows. It held focus groups and showed people prototypes of the tile interface and its live updates. “We would get this delightful reaction of people who would say, ‘This is so great, and it has Office too,’” said Jensen Harris, Microsoft’s director of program management for the Windows user experience.

Sixteen million people have been using early versions of the software. The boldness of the changes has delighted some users, who say they believe that for the first time, the company is taking greater creative risks than its more celebrated rival, Apple.

“I think it’s functional, clean,” said Andries van Dam, a pioneer in computer graphics and a Brown University computer science professor, who receives research money from Microsoft. “I welcome it.”

Younger users may be more likely to embrace the new approach. Joanna Lin, 23, who works in sales and marketing for a hotel chain in New York, said she was impressed with the software. “The feeling was very fluid,” said Ms. Lin. “Definitely a step up from Windows 7.”

But the product is a major gamble for Microsoft, a company whose clout in the technology industry has been waning. The PC business, which generates much of Microsoft’s revenue, is in a severe slump as newer products like smartphones and tablets take more dollars from peoples’ wallets.